Improve exec command processing

- Make exec print out an error if execve() failed for some reason
- Pass init's environment, created by the export keywords, to the exec'd
  command, which is helpful when running shell scripts (for PATH) and/or
  am (for BOOTCLASSPATH) from an exec statement.
Make bootmenu provide its own full libreboot.
Merge remote-tracking branch 'upstream/gingerbread' into gingerbread